The bulldozer includes a work implement, an engine, a torque converter, a travel device, and a power transmission unit. The power transmission unit is configured to transmit engine output to the travel device via the torque converter. The travel device is composed of crawler belts (see Herbert L. Nichols, Jr. et al., “MOVING THE EARTH,” 4th edition, p 15.1-15.9, The McGraw-Hill Companies, 1998, United States).
The bulldozer excavates the ground while pushing soil with the work implement (e.g., blade). When a large load acts on the vehicle in the excavation work of the work implement, the torque converter enters a completely/nearly stalled state. The term “completely stalled state” refers to a state that a speed ratio of the torque converter is zero. On the other hand, the term “nearly stalled state” refers to a state that the speed ratio of the torque converter is roughly zero.